Semiconductor gas sensors function by offering a change in electrical resistance in response to a shift in the local concentration of the gas of interest. In general the resistance of the sensor is some function of the concentration of the target gas. It is generally preferred that the semiconducting materials used should offer a response, which is selective for a particular gas, without interference from other components of the atmosphere, including moisture (relative humidity).
The mechanism operating at temperatures within the range 200-500° C. involves reactions of the molecules of the target gas at the surface of the semiconductor (which is usually a metal oxide) and results in a change in near-surface charge-carrier density. Examples of materials functioning through this mechanism can be employed in the detection and monitoring of either reducing gases (hydrogen, carbon monoxide, methane etc.) or oxidizing gases (nitrogen dioxide, chlorine, ozone etc.) in ambient air. The sign of the response (a resistance increase or a resistance decrease), depends on whether the semi-conducting material used is n-type or p-type.
The present applicant has shown that sensors composed of W0.9Mo0.1O3 provide excellent n-type responses (resistance increases) to the oxidizing gases ozone and NOx with only minimal interference from changes in relative humidity. Applicant has also shown that chromium oxide (Cr2O3), when doped with a few percent of titanium dioxide, provides p-type responses to the reducing gases, ammonia and hydrogen sulfide (also resistance increases) with little interference from changes in relative humidity.
Of course, air quality can be degraded by the presence of oxidizing gases, by reducing gases, or by both. Any one sensor can only provide information about the presence of either reducing gases or oxidizing gases, however, because the sign of the sensor response to each type is different: a resistance increase in one case and a resistance decrease in the other.
Needs exist for simple detectors that can detect degradation of air quality and provide single indications regardless of the gas causing that degradation.